Republican US Senate candidate Richard Mourdock urges his supporters to get to the polls on Tuesday as he makes a campaign stop Monday at Immanuel Reformed Presbyterian Church near Battle Ground, Ind. Mourdock is running against incumbent Sen. Dick Lugar.

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Democrats in Indiana, not to mention Washington, D.C., are abuzz over the likelihood that the Hoosier State could soon – and unexpectedly – be added to the list of competitive US Senate races in 2012, boosting Democrats' chances of retaining control of the upper chamber of Congress.

If Sen. Richard Lugar (R) goes down in Tuesday's GOP primary as a result of an attack from his right, a Democrat will face the tea-party-backed Richard Mourdock rather than the six-term senator with a reputation as a statesman and a Republican moderate. And Democrats like their chances in that match-up much better.

By no means would the Democratic candidate, expected to be US Rep. Joe Donnelly, be a shoo-in. But at least he would have a fighting chance. A hypothetical matchup between Mr. Mourdock, Indiana's treasurer, and Mr. Donnelly puts the candidates neck and neck, at 35 percent each, according to a Howey/DePauw poll in late March. By contrast, Senator Lugar would trounce Donnelly by 21 percentage points, the same poll showed.

“The battleground is pretty good” for a Donnelly victory, says Brian Vargus, a political scientist at Indiana University-Purdue University in Indianapolis. “The best way to think of it is that it is a completely unanticipated possibility for Democrats to pick up a [US Senate] seat.”

Lugar himself, in the waning days of the campaign, began to warn Indiana voters of what a Mourdock win could mean for the state GOP.

“Democrats understand Joe Donnelly will beat Richard Mourdock. This is serious," Lugar wrote in an e-mail blast to supporters on Sunday. "Losing our Indiana Senate seat to the Democrats is not a risk that Republicans can take.”

A sizable number of Republicans, however, seem poised to take their chances. Mr. Mourdock has picked up endorsements from Sarah Palin and others on the conservative side of the party, and his campaign has benefited from millions spent on ads by national conservative groups such as the Club for Growth, the National Rifle Association, and FreedomWorks. Those ads have attacked Lugar as too moderate and too willing to compromise with President Obama. Also at issue is Lugar’s acknowledgement that he no longer permanently resides in Indiana, but spends most of his time in Washington.

Pummeling Lugar with negative ads on those themes is working, recent polls show. Lugar trailed Mourdock by 10 percentage points, according to a Howey/DePauw poll released May 1.

Lugar complains of being outspent by the "super political-action committees" that back Mourdock’s campaign and only recently launched his own negative ads criticizing his opponent’s record.

Donnelly, sensing GOP vulnerability, has turned his attention to Mourdock in recent days. In a speech last week in Indianapolis, Donnelly reserved most of his remarks for an unsuccessful lawsuit in 2009 to stop the federal government’s bailout package for automaker Chrysler.

“It was forced by Richard Mourdock, who wouldn’t stand up for Hoosier workers,” Donnelly said. Mourdock’s partisanship can be blamed for taking his state “to the brink of bankruptcy,” he added. Lugar's name never came up.

Donnelly’s congressional district includes South Bend and Michigan City, which lean Democratic (though Indiana Democrats are more conservative than Democrats as a whole). Donnelly, if he wins the Democratic nomination, is likely to pound the theme that Mourdock's brand of conservatism is too harsh for Indiana, says Andrew Downs, director of the Mike Downs Center on Indiana Politics at Indiana University-Purdue University Fort Wayne. That may play well in the Rust Belt section of northwest Indiana, which benefited from the auto bailout, and with independents and moderate Republicans.

Democrats “would much rather face Richard Mourdock because he painted himself as very conservative, and they are going to do nothing to dispel that,” says Mr. Downs.

Winning his party’s nomination Tuesday would force Mourdock to consider scaling back the tea party messaging of his campaign.

“If Murdock is able to bring the [Republican] Party back together and is able to moderate his position to some degree to bring in some less strident Republicans back to the polls, he could end up winning,” Downs says.

Outside a poll Tuesday in Indianapolis, Mourdock said he felt “the race has national implications regarding the direction of the Republican Party,” according to a video from the Indianapolis Star.

“There are places in the country," he said, "where there are tea party activists who are thinking, ‘If it can happen in Indiana, it can happen here.’ ”